In order to fullscreen a chart within Coinigy, find the icon in the row of tools above said chart that depicts four arrows pointing away from the middle of the icon box like so:
Clicking this icon will instantly fullscreen the chart(s) you are currently viewing. You can go back to your regular view by pressing the ESC key on your keyboard.
Alternatively, if you would just like to enlarge the chart rather than full screening it, clicking the arrows on either side of the chart (as depicted below) will collapse the side panels and make your current chart larger. Likewise, when the side panels are collapsed, clicking Data and Trading or Markets will make the respective side panel return to view.